1. Field of the Invention
The present invention relates to a portable, combustion-engined tool, in particular a setting tool for driving in fastening elements and including a combustion chamber an inlet/outlet valve of which is closed or opened dependent on an operational phase of the tool, and to a method of controlling the operation of such a tool.
2. Description of the Prior Art
In the tool described above, a drive energy is obtained by combustion of a fuel gas mixture and is transmitted by a piston to a fastening element. By pressing the tool against an object in which the fastening element is to be driven in, an ignition of the fuel gas mixture in the combustion chamber is initiated. The initiation of the ignition takes place upon actuation of an ignition device by a trigger which is actuated upon the tool being pressed against the object. The ignition device produces an electrical spark that ignites the fuel gas mixture, starting a combustion process. The increased pressure, which is produced by the combustion of the fuel gas mixture, acts on the piston which adjoins the combustion chamber, driving the same in the setting direction. At the end of its displacement in the setting direction, the piston passes past outlet openings which are formed in a guide cylinder, in which the piston is located, and through which opening exhaust or waste gases can be at least partially removed. The piston then returns to its initial position as a result of an underpressure created in the combustion chamber as a result of cooling down of residual gases still remaining in the combustion chamber. During the time the piston returns to its initial position, the combustion chamber should remain sealed from the surrounding environment. The inlet/outlet valve, through which fresh air enters the combustion chamber, should open only after the return movement of the piston has been completed. Generally, the time necessary for return of the piston to its initial position, increases with an increase of the tool temperature. In addition, a high-energy tool requires that a large expansion volume be available, which also increases the time of the return movement of the piston.
In conventional tools, the inlet/outlet valve can be closed with an appropriate latch fixedly connected with a trigger by a toggle lever. The inlet/outlet valve becomes open as soon as the trigger, which is associated with the piston, returns to its initial position.
The locking of the inlet/outlet valve with a trigger means that the shift point of the trigger cannot any more be arbitrary selected. The ignition switch can only then be actuated when the closing of the inlet/outlet valve has been completed, i.e., much later after the trigger movement. However, a prolonged trigger movement adversely affects or influences the customer acceptance of such tools. Further, with a hot tool, the return movement of the piston, as it has already been discussed above, takes more time. In this case, the user has to hold the trigger in its pulled position much longer in order to prevent the piston from occupying an erroneous position.
Naturally, in order to increase the time during which the piston returns to its initial position, the movement of the trigger can be damped. However, damping of the trigger movement adversely affects the trigger characteristics as the triggering force is increased, and the trigger itself does not return to its initial position sufficiently rapidly. Users view dampening of a trigger very unfavorably as it reduces the output and increases actuation forces that need be applied by a user.
A further non-insignificant problem consists in that not in each case, return of the piston to its exact initial position is insured.
Accordingly, an object of the present is a tool of the above-described type and a method of controlling its operation which would insure a complete return of the piston to its initial position before the inlet/outlet valve opens, without any manipulation of the trigger by a user.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a tool having blocking means providing for closing and opening of the inlet/outlet valve, and means for controlling the operation of the blocking means in accordance with the pressure in the combustion chamber; and by providing a method according to which the closing and opening of the inlet/outlet valve is effected dependent on the gas pressure in the combustion chamber.
In this way, the closing of the inlet/outlet valve is automatized and, in addition, ignition of the gas mixture in the combustion chamber takes place independent of the displacement position of the trigger. In this way, it is insured that the piston always returns to its initial position before opening of the inlet/outlet valve. The operation is effected completely automatically, without intervention of the user, in particular, because opening of the inlet/outlet valve is not any more controlled by the trigger movement.
As soon as an overpressure is produced in the combustion chamber after ignition of the fuel gas mixture therein, at least one inlet/outlet valve can be closed (if the combustion chamber has several inlet/outlet valves). This can in principle take place at any overpressure as closing of the inlet/outlet valve(s) alone is not absolutely necessary for displacement of the piston. However, closing should take place in each case after the overpressure in the combustion chamber has been created and the piston has been displaced, as now retaining of the underpressure, which is necessary for return of the piston to its initial position as a result of cooling of the residual gases, should be insured. The combustion chamber, in this case, should not be aerated, and the inlet/outlet valve should remain closed, which requires locking the valve in its closed position. Otherwise, a complete return of the piston to its initial position would not have been possible. To unsure this, closing already starts after the gas pressure have reached a predetermined, relatively small value.
Opening of the inlet/outlet valve takes place after expiration of a predetermined time period after overpressure in the combustion chamber has been produced. The predetermined time period can, e.g., be determined based on previous empirically determined data. At that, the time, necessary for return of the piston to its initial position, should lie within the predetermined time period. It proved advantageous to count the predetermined time period starting from the point the maximum gas pressure in the combustion chamber has been reached, as a maximum gas pressure can be easily achieved in the combustion chamber.
According to the present invention, as soon as the gas overpressure has been detected, somewhat shortly after the beginning of the ignition process, the closing takes place. Then, the time-delay element, which provides for automatic opening after a predetermined time period, is actuated. This time period, as it has already been discussed above, is so selected that opening starts after the piston has been completely returned to its initial position. The return of the piston to its initial position can take place independent of the actuation of the trigger which insures a reliable operation of the tool.
The blocking device can be formed in any arbitrary manner. It only should be insured that it reacts to the inner pressure of the combustion chamber. When the blocking device is formed as an electro-mechanical or purely electrical device, for determining the inner pressure in the combustion chamber, a pressure/voltage transformer can be used. According to the invention, the blocking device can include a pneumatic device, in which case, its operation is initiated directly by the pressure in the combustion chamber.
According to preferred embodiment of the present invention, the blocking device includes a check valve connected with the combustion chamber, and a pneumatic cylinder located downstream of the check valve, with the conduit connecting the check valve with the pneumatic cylinder communicating with the surrounding environment via a throttle. The check valve only permits gas flow from the combustion chamber to the pneumatic cylinder for actuating a piston having a piston rod and displaceable in the pneumatic cylinder. After the over pressure in the combustion chamber reaches its maximum and is them reduced, the check valve closes automatically as the pressure in the conduit, which connects the check valve with the pneumatic cylinder, is greater than in the combustion chamber. The conduit, together with the throttle, forms the time-delay element as the pressure in the conduit is gradually reduced by the throttle. As the pressure in the conduit is reduced, the piston is biased to its initial position by a compression spring located in the cylinder, releasing the blocking member which results in opening of the inlet/outlet valve. The speed of the movement of the piston in the cylinder and the release of the inlet/outlet valve can be selected by adjusting the throttle, whereby the predetermined time period, which is determined by the time necessary for return of the piston into its initial position can be matched to corresponding environmental conditions and/or constructive features of the tool.
The piston rod of the piston of the pneumatic cylinder can, e.g., pivot the blocking member into the displacement path of a drive ring, which is arranged outside of the combustion chamber, for blocking the movement of the drive ring or releasing the drive ring which actuates the inlet/outlet valve. Due to the use of the pivotal blocking member, the dimensions of the entire construction are only slightly increased.
The present invention can be used in tools having a single-volume combustion chamber. However, the invention can also be used in tools having a multi-sectional combustion chamber the chamber sections of which are separated by one or several separation wall(s) or plate(s) provided with a plurality of through-openings. In the later case, the pressure in the chamber section adjoining the piston controls the closing and opening of the inlet/outlet valve or valves. The present invention can also be used with tools having collapsible combustion chambers which include a plurality of chamber sections separated by movable walls which are pushed onto each other during deaeration of the combustion chamber, which results in collapse of the combustion chamber. As long as the aeration/deaeration valve(s) remains closed, and as long as the piston has not yet returned to its initial position, the collapse of the combustion chamber should be prevented, so that the same pressure conditions, which control closing/opening of the inlet/outlet or aeration/deaeration valve, control the locking or release of the movable walls of the combustion chamber.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objets thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.